Technical Field
The present invention relates to a delivery system with a self expanding stent for implantation into a blood vessel, especially in the region of the aortic arch, said stent comprising a hollow cylindrical body which is radially compressed for implantation, and with a pull-back sheath which surrounds the stent and which radially compresses the latter for positioning and releasing the stent in the blood vessel.
Background Art
Delivery systems of this kind are used to implant endovascular stents for treatment of aneurysms in arteries. An aneurysm is understood as a widening or bulging of an arterial blood vessel as a consequence of congenital or acquired lesions of the vessel wall. The bulge can affect the vessel wall as a whole or, in what is called a false aneurysm, blood can flow from the lumen of the vessel in between the layers of the vessel wall and can tear these apart from one another. Nontreatment of an aneurysm may lead to a rupture of the artery in advanced stages, after which the patient may suffer internal bleeding.
Although aneurysms often occur in the area of the abdominal aorta (aorta abdominalis) or thoracic aorta (aorta thoracica), an aneurysm may, however, also occur in the area of the ascending or descending branch of the aorta (aorta ascendens and aorta descendens). The ascending branch of the aorta is directly connected to the heart. Starting from the aortic root (sinus aortae), the ascending branch extends upward in a slightly curved shape away from the heart and merges into the aortic arch (arcus aortae). The vessels of the head, among others the left and right carotid arteries, branch off in the area of the aortic arch. The aortic arch follows a curve of approximately 180 degrees with a very narrow radius and connects the ascending branch of the aorta to the descending branch.
The stents used for treating aneurysms of this kind comprise a hollow cylindrical metal frame whose circumferential surface is covered with a textile film or polymer film so as to provide a hollow cylindrical body. For implantation, the stent is radially compressed such that its cross-sectional surface area greatly decreases. With the aid of a delivery system, the stent is then introduced into the area of the aneurysm, where said stent is released. By virtue of the resiliency of the metal frame, the stent expands again into its original shape and thus braces its circumferential surface, the latter fastening itself in the inside of the blood vessel at positions proximal and distal to the aneurysm. In this way, the blood now flows through the stent, and further stressing of the bulge is prevented.
To obtain the desired effect of the stent, it is not only necessary to position the latter axially in such a way that it can brace itself at positions distal and proximal to the aneurysm in the relevant blood vessel; the radial orientation of the stent is often also of critical importance. This is especially the case when, at positions proximal to the aneurysm, other vessels branch off from the blood vessel affected by the aneurysm, as is the case for example in the region of the aortic arch where the arterial vessels of the head branch off. To ensure that the blood supply to these branching-off vessels is not impaired, the stents are often provided with lateral openings through which blood can pass from the interior of the stent. These openings have to be positioned in the area of the origin of the branching-off vessels, for which purpose it is necessary not only to move the stent in its longitudinal direction during its implantation, but also to rotate it about its longitudinal direction.
For implantation, these stents are radially collapsed and are then inserted into the blood vessel with the aid of intraluminally advanced catheters and positioned at the correct location in the region of the aneurysm. The correct location of the stent can be monitored via X-ray markers which are provided on the jacket of the stent, in particular in the area of the openings for supplying blood to the branching-off blood vessels.
To ensure that the stents remain in the collapsed state during the positioning procedure, they are arranged in a sheath or a tube which presses the stent radially inward. This so-called pull-back sheath is withdrawn after the stent has been positioned in the region of the aneurysm, the stent being held axially by a stop tube which is also referred to as a pusher. The pusher is in contact with the stent and maintains it in its axial position, while the pull-back sheath also surrounding the pusher is removed from the stent, the latter then expanding and bracing itself in the blood vessel.
However, the delivery systems described thus far do not work satisfactorily in those applications in which the stent has to be positioned in a curved section of a blood vessel, for example in the aortic arch.
The stents used here have particularly large dimensions; even in the radially compressed state, i.e., the collapsed state, they still have a diameter of 6 to 8 mm. The pull-back sheaths used for this consist of polymer tubes which are in most cases made from polyethylene or tetrafluoroethylene. The wall thickness of these polymer tubes is dimensioned such that it withstands the expansion pressure of the collapsed stent, remains stable over the course of time and is not subject to any thermal creep. This means, however, that the pull-back sheath has a relatively high geometric moment of inertia of its cross-sectional profile. Moreover, the pull-back sheaths are relatively rigid, so that the operating surgeon does not lose control of the degree of stent release.
If delivery systems of this kind, that is to say with radially collapsed stents placed in a rigid pull-back sheath, are implanted in narrow vessel radii, such as in the aortic arch, the pull-back sheath tend to form kinks as a result of their substantial geometric moment of inertia. One or more such kinks in the pull-back sheath then jam in the collapsed stent when an attempt is made to release it and they make it difficult or even impossible to achieve a complete release in the vessel arch at the desired location.
When delivery systems of this kind are used, the operating surgeon therefore draws the delivery system back into a distal and therefore straighter vessel region after implantation, in order to be able to release the stent there to a certain degree. Thereafter, the partially deployed stent is inserted back into the vessel arch, which is a particularly risky maneuver, as there is a danger of perforating the vessel wall. Once the stent has been correctly positioned again, it is then released completely, with considerable force being applied.
In addition to the risk of perforating the vessel wall, this maneuver often has the result that the stent cannot be positioned with adequate precision. The substantial force which has to be applied in order to overcome the kinks in the pull-back sheath also contributes to this.